Posted: 18/08/2014 at 20:39
philippa - we had a similar situation at my last house. Huge pond which had large koi and golden orfe in it which had been overfed fro years, so lots of excess debris, but it was essentially a wildlife pond. We renovated a lot of it and improved it enormously, fed the fish less, and just tried to achieve a balance for everything in it. Masses of frogs, toads and dragonflies and loads of other wildlife in and around it. We couldn't have cleared it out without spending a fortune and several months of hard graft, and possibly ruining what was already there.
I hope you can alter your pond Doreen and make it right for you. It's well worth it for the pleasure it brings, to you and the wildlife. The deeper you can make it the better - even if it's just in a small part of it, with a graduating slope for access. It helps to prevent it heating up too rapidly in spring and freezing solid in winter. I've had two ponds which haven't been any deeper than 18" at their deepest point and they've been fine, but it does depend on other factors like planting and location.